1. Field of the Invention
The present invention relates to electronic warfare (EW) systems that use polarimeters to measure polarization of a received radar signal and to transmit electronic counter measures (ECM) signals with a preferred polarization and, more particularly, to a digital implementation of such a polarimeter.
2. Description of the Related Art
Ground-based and airborne radar systems have long been used to detect and track hostile targets, such as aircraft and missiles. Typically, a radar system transmits RF signals, such as a sequence of pulses, toward the target. The surface of the target reflects at least some portion of the incident signal energy back toward the radar antenna where the pulse echo is detected, allowing the radar system to determine the target's range, angle and velocity.
To counter the effectiveness of such radar systems, electronic warfare (EW) systems attempt to interfere with radars signals by generating electronic counter measures (ECM) signals designed to confuse, mislead or overwhelm the radar system. By accurately determining the polarization of an arriving radar signal, an EW system can transmit a signal with the same polarization back to the radar system to interfere with the echo signal, or transmit a signal with an orthogonal polarization to deceive or jam the radar system. EW systems have relied on polarimeters to accurately measure the polarization of received radar signals and to control the polarization of the ECM signals transmitted back to the radar system.
In systems presently in operation, only one polarimeter is typically used. The polarimeter processes low power level signals received from the radar and imparts a desired polarization to the high power RF signal from the EW system transmitter. To operate on the transmitter output signals, the polarimeter design must be based on hardware components with a capacity to handle high power. Thus, conventional polarimeters are bulky, heavy and costly devices that dissipate as much as fifty percent of the RF signal power, adversely affecting the EW system performance. Consequently, the traditional polarimeter design is not affordable for many small or low-cost EW systems.
With the more recent advent of power amplification technology devices, i.e., Microwave Power Modules (MPM) and Gallium Arsenide Monolithic Microwave Integrated Circuits (GaAs MMIC) chips for active aperture antennas, new analog hardware architectures for future EW systems could be contemplated. In the new architecture, the single bulky polarimeter is replaced with two MMIC low-power polarimeters. A receive polarimeter, involving a four terminal network, is inserted into the receiver line and measures the polarization of intercepted signals. A transmit polarimeter is positioned at the input to a pair of power amplifiers and controls the polarization of the transmitted signal based on measures from the receive polarimeter. In the receive polarimeter, the polarization measurement accuracy must be very high in order to achieve a high level of jamming performance, with tolerable errors being less than 0.5 RF degrees. A null adaptive tracking technique accurately measures the received signal's polarization by phase shifting the vertical and horizontal polarization components of the signal and developing sum and difference measurement from combinations of the phase-shifted components. The phase shifts producing a minimum or “null” difference/sum ratio identifies the polarization of the received signal. Performance of the null adaptive tracker is rated in terms of an achieved null depth, with a deeper null representing greater performance.
While low-power polarimeters employing a null adaptive tracking technique overcome some of the disadvantages associated with high-power polarimeters, the low-power polarimeter design is still based on the architecture of analog components. Analog polarimeters typically employ hybrid junctions for combining the received signal components. These hybrid junctions may permit some degree of RF cross leakage to corrupt the combined signals, thereby complicating the task of achieving a deep null and limiting performance of the analog polarimeter. Moreover, the feedback mechanism within the null adaptive tracker of the analog polarimeter requires a significant amount of time to reduce the difference signal to zero, since fresh signal samples (i.e., pulses) are required for each iteration performed by the tracker.
While a polarimeter based on digital technology could potentially avoid these problems, the extraordinarily high digital sampling rates that would be required to accurately measure polarization are not feasible with current technology. Accordingly, there remains a need for a reliable, inexpensive and compact polarimetric system that overcomes the performance limitations of analog systems using currently available digital technology.